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Transposable elements (TE) are selfish genetic elements that can cause harmful mutations. In Drosophila , it has been estimated that half of all spontaneous visible marker phenotypes are mutations caused by TE insertions. Several factors likely limit the accumulation of exponentially amplifying TEs within genomes. First, synergistic interactions between TEs that amplify their harm with increasing copy number are proposed to limit TE copy number. However, the nature of this synergy is poorly understood. Second, because of the harm posed by TEs, eukaryotes have evolved systems of small RNA-based genome defense to limit transposition. However, as in all immune systems, there is a cost of autoimmunity and small RNA-based systems that silence TEs can inadvertently silence genes flanking TE insertions. In a screen for essential meiotic genes in Drosophila melanogaster , a truncated Doc retrotransposon within a neighboring gene was found to trigger the germline silencing of ald , the Drosophila Mps1 homolog, a gene essential for proper chromosome segregation in meiosis. A subsequent screen for suppressors of this silencing identified a new insertion of a Hobo DNA transposon in the same neighboring gene. Here we describe how the original Doc insertion triggers flanking piRNA biogenesis and local gene silencing. We show that this local gene silencing occurs in cis and is dependent on deadlock , a component of the Rhino-Deadlock-Cutoff (RDC) complex, to trigger dual-strand piRNA biogenesis at TE insertions. We further show how the additional Hobo insertion leads to de-silencing by reducing flanking piRNA biogenesis triggered by the original Doc insertion. These results support a model of TE-mediated gene silencing by piRNA biogenesis in cis that depends on local determinants of transcription. This may explain complex patterns of off-target gene silencing triggered by TEs within populations and in the laboratory. It also provides a mechanism of sign epistasis among TE insertions, illuminates the complex nature of their interactions and supports a model in which off-target gene silencing shapes the evolution of the RDC complex. 

Efforts to broaden participation in computing address how systemic school structures, educator preparation, and curriculum can provide inclusive learning spaces for all students. The emerging multiplicity of scholarship in computer science (CS) education forwards diverse voices, perspectives, and positionalities, and together, provide a rich set of evidence-based narratives that can transform K-12 policies and practices. The four projects featured in this panel bring together CS education efforts with varying methodologies focused on equity-oriented pedagogies and learning for all youth across the US. This panel will focus not only on sharing the multi-pronged efforts of the featured projects, but also on developing a shared vision among participants and panelists for what equity" can and should be in the future of both SIGCSE and CS education as we celebrate SIGCSE's 50th anniversary. By highlighting the work of projects rather than individuals in this panel, audience members will have the opportunity to learn about how collaborative efforts create and examine contexts for equity in CS education across diverse stakeholders, while also providing a richer base for constructing visions of equity that go beyond mere platitudes, toward action items for broadening participation in computing. 

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressingE. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals  <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data.